<asp:CheckBox id=chkBInfo runat=server checked=true/>

<input id="ctl00_ContentPlaceHolder1_chkBInfo" type="checkbox" name="ctl00$ContentPlaceHolder1$chkBInfo" checked="checked" />

function theForm_onSubmit(){

        if (document.forms(0).<%=chkBInfo.ClientID%>.checked==false)

        {

            alert('Please select a Message Type...');

            return false;

        }

}